The present invention generally relates to improved oxygen barrier liner compositions for plastic closures and more particularly, to homogeneous thermoplastic elastomer liner compositions which provide an effective barrier to oxygen ingress into the containers and which are advantageously characterized by improved physical properties such as increased tensile strength and elongation. This invention also relates to a method for making such liners from such plastic compositions and to closures for containers for food and beverage products which closures include these liner compositions.
Closures for use in food and beverage containers include a closure shell formed of metal, plastic or both metal and plastic and are typically provided with a liner on the inner surface of the closure shell end panel. The liner is intended to provide a hermetic seal between the closure member and the container opening.
Notwithstanding the lined closure, oxygen can permeate the closure shell or enter through spaces between the closure shell and the container. Oxygen can adversely affect beverage and food products stored within a container since a small amount of oxygen can alter the taste of the beverage or food product or cause spoilage of the product. Accordingly, it is desirable that the liners be made of or include a material that is a barrier to oxygen. Efforts to provide a liner that is an effective barrier against oxidation of the food or beverage stored within a container are described in the prior art.
U.S. Pat. No. 5,955,163 to White discloses a gasket for closures used with beverage containers. The gasket is formed of a thermoplastic material which includes, for example, a hydrogenated copolymer of styrene and conjugated diene or functionalized derivative thereof and a rubbery polymer such as butyl rubber which, according to this patent, further prevents ingress of oxygen and volatile odors. The gasket is used with metal crown caps.
U.S. Pat. No. 4,684,554 to Ou-Yang describes a multi-layered seal that includes a pulpboard backing, a wax coating over the pulpboard, aluminum foil and a heat-sealable polymeric coating over the foil. The aluminum foil acts as an oxygen barrier. The multi-layered seal is mounted inside the closure of a container. The container and the closure (with the seal) is treated in a radio frequency field, such that when the closure is removed from the container, the pulpboard backing twists free from the foil which remains bonded to the lip of the container. The foil is then peeled off the lip of the container when access to the contents is desired. While aluminum foil provides a good barrier to oxygen, from the standpoint of a consumer, such peelable seals are less than desirable because of the difficulty often encountered in removing them from the opening of the container.
Another example of a closure with an oxygen absorbing liner is described in U.S. Pat. No. 5,381,914 to Koyama et al. The liner described therein is made of a resin which is blended with an oxygen absorbing agent. The composition can be applied to the entire inner surface of the closure shell or, more typically, may be incorporated as a layer of a laminated structure. The laminate includes the layer of the oxygen absorbing agent and a layer of resin interposed between the oxygen absorbing layer and the closure shell. The oxygen absorbing layer can be made of a thermoplastic elastomer with the oxygen absorbing agent blended therein. Examples of suitable organic oxygen absorbing agents include phenol-type resins, ascorbic acids, saccharides and the like.
U.S. Pat. No. 5,143,763 to Yamada et al. describes a multi-layered seal for use with bottle closures that include a layer of an oxygen absorbent composition and an oxygen permeable film covering the oxygen absorbent composition. The composition includes an asymmetric porous membrane where the outer surface is formed as a dense skin layer. The dense skin layer protects the oxygen absorbent layer from being contacted by the food or beverage stored within the container.
While the liners or seals described above may be effective in limiting the amount of oxygen ingress into the container, further improvements in the field of oxygen barrier liners for closures are desirable.
For example, it would be desirable to provide a liner which acts as an oxygen barrier and can be used in association with resealable plastic closures. It would also be desirable that such liners not require peeling or removal by the consumer.
In addition to providing a barrier to oxygen, liners for use in food or beverage container closures should possess other properties. For example, the liner and the plastic compositions used for such liners must possess good to excellent adhesion to the inner surface of the closure preferably without the use of a separate adhesive.
Also, the sealing provided by the liner should not be such that it is difficult for the average user to open the container. Accordingly, the liner must have good xe2x80x9ctorque removalxe2x80x9d properties to allow for easy removal (e.g., by unscrewing) of the cap without having to use excess force.
The liner should also be made of a composition that is easy to process. Oil-based plasticizers, such as mineral oil, have been included in liner compositions to improve the processability of the composition. However, under certain conditions, extractable compounds from the mineral oil have been known to migrate from the liner composition to the stored food product. Plasticizers such as mineral oil have also been known to exhibit an odor and/or affect the taste of the food product. Thus, it would be desirable to provide a liner that is easy to process but (1) does not include an oil or other plasticizer, (2) does not affect the taste of the stored food product, and (3) is substantially odorless.
It would also be desirable to provide a liner that can be easily formed into flat liners by known techniques such as injection molding and cold punch molding, and that can otherwise be easily incorporated into the closure.
The plastic composition of the present invention and liners made from such plastic compositions address at least all of the above-described objectives. For example, our studies have shown that the preferred single layer liner in accordance with the present invention has an oxygen ingress rate of between approximately 4-8 cc/m2/day, a 100% modulus greater than approximately 260 psi, % elongation of greater than approximately 350 and a tensile strength (psi) greater than approximately 500. Also, we have observed that the desirable properties described above can be affected by the relative proportions of the compounds used in the liner composition. For example, excess polybutylene will negatively affect the sealability of the liners in relation to the container opening, whereas too little polybutylene can reduce the adhesion of the liner to the inner surface of the closure cap. An amount of polyisobutylene that is significantly lower than the amount used in the composition of the present invention will reduce the oxygen barrier properties, while too much polyisobutylene can make the composition more difficult to process.
There are several different aspects to the present invention. In one aspect, the present invention is directed to a closure cap liner or gasket composition comprising a blend of a thermoplastic elastomer, polybutylene and polyisobutylene. In another aspect of the present invention, the plastic composition can also advantageously include a microcrystalline wax.
Plastic compositions of the type described above exhibit excellent oxygen barrier properties and, therefore, are useful in closures for food or beverage containers. Accordingly, in another aspect, the present invention is directed to a container closure comprising a plastic shell having an end panel and an integral skirt that extends downwardly from the periphery of the skirt. The end panel has an inner-facing surface and includes a substantially oxygen impermeant liner which is adhered to at least a portion of the inner-facing surface. The liner is made from a material including a thermoplastic elastomer, polybutylene and polyisobutylene. In another aspect of the present invention, the liner can also advantageously contain a microcrystalline wax.
The present invention is also directed to a method for providing a liner for a container closure. The method includes (1) combining and mixing polyisobutylene with a thermoplastic elastomer, (2) adding polybutylene to the mixture of polyisobutylene and thermoplastic elastomer to provide a blend, and (3) forming the blend into a liner. A microcrystalline wax can be advantageously added to the blend of thermoplastic elastomer, polybutylene and polyisobutylene. The liner can be formed into a disc or a ring adhered to the inner-facing surface of the closure.
The objects of this invention are achieved with closure caps that include liners made from a composition that comprises a thermoplastic elastomer, polyisobutylene and polybutylene.